Summary: Find out about cable sizes for electrical use in the UK and 1.5mm and 2.5mm and other Electric cables and the current they carry. Also learn about what electric flexes and electric wires do and protected against overloading. Discover the different types of electrical flexes and cables that might be used in your home's wiring and how to protect them against overheating under loft or wall insulation. Additionally, discover What the building regulations and IET regulations say about the current ratings in electrical cables and find out what rules and regulations you should work to.

Please remember when attempting any electrical installations at home that you are obliged to get the completed job tested by a fully qualified electrician and obtain a minor works certificate. Failure to do this may render your house insurance invalid and you may have difficulty selling your home.

Cables for Lighting Circuits

We get asked a lot which cable should be used to supply a lighting circuit. In domestic situations this is usually a 1.5mm cable. For what 1.5mm cable means, see below. If you are interested in purchasing energy saving light bulbs, they are available here.

Lighting is usually fed by 1.5mm cable

Cables for Sockets

In domestic situations the usual cable used for sockets on a ring main or radial main is 2.5mm cable. For an explanation of the 2.5mm cable please see below.

Sockets now carry USB outlets

Why are Different Cables Used for Different Things?

It is vital to remember that values for cables and flexes can change in domestic situations. A cable in an insulated loft space will get hotter, much more quickly, than a cable looped through garage rafters. The suggested load it can carry is then less. Loading a cable with too many amps makes it hot and hot cables are dangerous.

The Institute of Engineering and Technlogy (IET), formerly the Institute of Electrical Engineers (IEE) have decreed that cables can carry loads (measured in amps) according to not just there size, but according to the methods they are fixed (or otherwise) to any given surface. These methods are described below.

As with most formulas in the building trade there are regulations defining specific boundaries for the use of all materials. Factors such as resistance and voltage drop may need to be assessed and taken into consideration when working out cable runs.

The cable current rating for different fixing methods table below shows the methods by which cables are employed in a domestic dwelling and the permissible voltage drop via that cable and that particular method.

IET - Institute of Engineering and Technology

Electrical Deaths and Accident Statistics

To show just how dangerous electricity can be, Electrical Safety First (a charitable organisation that campaigns, on behalf of consumers, for the upholding of and improvements on, all safety legislation in the UK) lists the accident and fatality statistics for all to see. We have published some statistics below.

Electrical safety first

A stunning fact is that around 2.5 million people in Great Britain receive electrical shocks each year and 350,000 of those suffer serious injury as a result. That's pretty scary on it's own, now read on!

Major Products Involved in Electrical Fires in 2011/12

Product

Fires

Injuries

Deaths

Cooking Appliances

11954

1477

10

Electricity Supply - Wiring, cables, plugs

2899

223

10

Washing Machines and Tumble Driers

1083

80

0

Lighting

767

74

1

Dishwashers

475

68

0

Number of Electrical Fires in UK for 2011/12

2011/12

All Accidental Electrical Fires

Accidental Domestic Fires of Electrical Origin

Faults

Misuse

Articles too Close to Heat

Total

Deaths

244

25

11

10

46

Injuries

5956

799

1392

278

2469

Fires

37601

7763

10735

1905

20403

Can I do my Own Electrical Work Indoors?

You are allowed to do some electrical work under Part P of the building regulations and our project page on Part P explains what you can and cant do. Even then it is suggested very very strongly that, to undertake any electrical work whatever you should be a “competent” person.

For the benefit of reason, it is as well to explain that competent does not simply mean you can walk, talk, read and write. It means that you must have the ability to test and electrical circuit for faults both before and after you have worked on it and be able to record and communicate the reasons for any difference in the readings.

Correct use of a multimeter is essential for safe electrical work at home

What is an Electrical Cable

The term cable, amongst other things, means "an encased group of insulated wires". A cable is a fairly inflexible (although of course they can be bent) set of wires used to supply the electricity to certain points in your home.

Your meter box is supplied through a cable, sockets are supplied by cables and your ceiling and wall lights are fed through cables.

A cable can carry many wires depending on the job it needs to do. Most domestic cables carry a blue wire which is usually for the neutral current, a brown wire for a live current and a bare wire to take residual current to earth. This cable is called 2 core and earth, or twin and earth.

The previous colours for live and neutral wires was red and black respectively. It is OK, when working on a building with the old colours, to joint the new to the old as long as the joint itself is done correctly.

For details of these changes please see the link to the Institute of Electrical Engineers. In essence, for domestic use, the cable wire colours will change to those of the flex colours.

Lighting circuits are usually fed by 1.5mm twin and earth cable

The bare wire, when the cable is used, should be marked by a green and yellow earth sleeve.

Another cable used a lot in domestic lighting is called 3 core and earth. The extra core (wire) is in an insulating sheath and is used as an extra conductor to carry power between 2 or more switches operating lights.

The brown wire, in a 3-core and earth cable should always be used as permanent live. The other two are interchangeable as long as they are sheathed in order to make it obvious which cable is neutral (blue sheathing) and which is the other live (brown sheathing).

Special lighting switch cable can be bought. This is called "Twin red core" and is used as switch cable for your lights. Often this is replaced, by electricians, who use an ordinary 2 core and earth cable as a switch cable and place a little red tape around the black wire in the cable. See our project on lights and switches.

Cable with 2 lives a neutral and an earth

Appliance Flex and why it is Used

Flex is short for flexible and the reality is that a flex is simply a flexible cable. Flexible cables are used for appliances because appliances usually get moved around a lot and the inflexibility of a fixed cable, either 1.5mm or 2.5mm, would soon crack the cable.

Appliance flex with live, nertral and earth

Why are Different Cables and Flexes Used

Different cables and flexes are used for different jobs because they are thicker and can carry more current and have more, or less resistance. Resistance can be thought of as electrical friction and the wires in the cable or flex will absorb some of the energy in the current, allowing a little less to reach the target than was sent.

High energy users such as electric showers, cookers and immersion heaters are supplied by thicker wires than (for example) radios, as the current that the appliance needs is considerably greater.

Cables supplying circuits etc should be sized as per the following cable ratings table. The methods mentioned in the table are methods for how the cable runs through the home to carry the power from the consumer unit to the outlet.

Cookers require high amperage as they use lots of power

Please note again that installation of cables depends on the location they are to be installed in. Also taken into consideration is the temperature of the area or void, the length of the run, the grouping of the points they serve and the type of device (Fuse, RCD etc) by which they are protected.

Cable Ratings and Locations From On Site Guide to BS7671:2008 Plus Ammendment 3 2015

We have abstracted a couple of the most popular tables for current rating of cables below. The rest, and the full table follows. The first table below is for cables which are installed by Method 103 “Surrounded by thermal insulation including in a stud wall with thermal insulation with cable not touching the wall”.

The second table is for cables installed by Method C, "clipped direct". As you can see there is quite a difference in rating so be absolutely sure you are doing the right thing.

Also remember that while it is not an offence to make electrical installations yourself, it is an offence to use them without them having been checked by a qualified electrician. Even a simple socket addition now has to have a minor works certificate.

The IET produce the On Site Guide to BS7671:2008+Ammendment 3 2015 and cable sizes are noted together with where they can go and which current they can carry. The mixture of these two things is called a Method and there are 7 methods. Their titles are:

Your house insurance may not be valid if you do not comply with the regulations as you have a duty of care to protect anyone who enters your home.

Books Making Electrical Wiring and Electric Lighting Easier

These books provides a great guide to wiring and lighting round the home. It should still be read with the knowledge that electricity is dangerous and you should check to ensure that you are actually allowed to do the work you would like.

What Does 1.5mm or 2.5mm Cable Mean?

1.5mm and 2.5mm is the cross sectional area of the individual live or neutral wires in the 1.5mm or 2.5mm cable. This is not the combined cross sectional area. Each one of the live and neutral wires has a cross sectional area of 1.5mm or 2.5mm.

1.5mm cable means a cross sectional area of 1.5mm of the live or neutral wire

If you snip one of the wires and look at one of the wires face down you will see it's cross section. Obviously this is a circle. The area, or cross sectional area of this circle is 1.5 square mm or 2.5 square mm.

2.5mm cable means a cross sectional area of 2.5mm of the live or neutral wire

This cross sectional area. or in more simple terms the area of the exposed face of the wire, does not include the insulation around it. It is purely the area of the copper wire.

How do I Calculate the Diameter of the Wire?

The calculation for the area of a circle is to multiply the radius (half the circle diameter) by itself to get the radius squared. This is then multiplied by Pi (Π) which is approximately 3.142. This calculation is written as shown in the image below.

Pi times the radius squared is the area of a circle

To calculate the diameter of the wire when knowing only the area, the equation has to be done backwards. The area is divided by 3.142 (Π) and the square root (√) of the answer is the radius (r) . Multiply the radius by 2 and you have the diameter.

What About the Size of the Earth Wire

The cross sectional area of the earth wire in a 1.5mm cable is 1mm and in a 2.5mm cable it is 1.5mm

Cable Size Ratings in Amps

Table 1 = Method 103

Surrounded by thermal insulation including in a stud wall with thermal insulation with cable not touching the wall.

Cable size

Rating in Amps

1mm

8

1.5mm

10

2.5mm

13.5

4.00mm

17.5

6.00mm

23.5

10.00mm

32

Table 2 = Method C - Clipped Direct

Cable size

Rating in Amps

1mm

15.5

1.5mm

20

2.5mm

27

4mm

37

6mm

47

10mm

65

Once again please check and double check that you are sure about what you are doing. It is an offence to install unchecked wiring and your house insurance may not be valid if you do.

The full cable rating table and Method descriptions (in accordance with 17th Edition of the Electrical Regulations) can be seen in the two tables below:

** In a stud wall with thermal insulation with cable touching the inner wall surface

Conductor Cross Section Area

Reference Method 103 (**)

Reference Method C* (***)

Reference Method A* (x)

Voltage Drop (xx)

1

5

6

7

8

(mm2)

(A)

(A)

(A)

(mV/A/m)

1

8

16

11.5

44

1.5

10

20

14.5

29

2.5

13.5

27

20

18

4

17.5

37

26

11

6

23.5

47

32

7.3

10

32

64

44

4.4

16

42.5

85

57

8.2

** In a stud wall with thermal insulation and cable touching the inner wall surface

*** Clipped direct

x Enclosed in conduit in an insulated wall

xx Per ampere per metre

Electricians Tools, VDE and a Word of Warning

Because of the danger of working with high voltages and high currents, electricians tools need to be completely and fully insulated. This insulation is usually safe up to 1000 volts.

The standards for electrical safety in electricians tools are set by a committee in Germany called the VDE or Verband der Elektrotechnik and they are very stringent as one would expect.

These rules make electricians tools quite expensive as a lot of work goes into them and a VDE approved screwdriver can cost as much as four times that of a normal screwdriver. Please, if you are doing and elevtrical work, make sure you have the right VDE approved tools for the job and do not be tempted to save money by buying cheap imitations which will not be anywhere near so well insulated.

(By submitting your email address you are agreeing to DIY Doctor Ltd using it to contact you with relevant information and offers. It will not be passed onto 3rd parties) Any questions should be posted to our Forum